Scientific curiosity is not always burdened by matters of great consequence. Over the years, considerable money and time has been applied to matters involving facial recognition between sheep, whether the flow of urine is impeded by someone watching you pee, and whether humans can capably swim in a pool full of syrup. (They can, almost as well as water.)
Now, researchers from Stanford University and Ecole Polytechnique in France have turned the roving eye of science to the phenomenon of knuckle-cracking. According to Gizmodo, a computer simulation was created to confirm an earlier theory that the audible noise that comes from the human hand after putting pressure on the knuckle was the result of gas bubbles popping inside the finger joint.
Conclusion: Probably true.
The study, published in Scientific Reports, demonstrated that microscopic bubbles inside the lubricating synovial fluid of the joint collapse when a knuckle-cracking session commences. To use an imperfect analogy, the cavitation bubbles are like the body’s Bubble Wrap. Popping them produces an audible—and for many, a very pleasing—sound.
To compile data, researchers took geometric representations of the joint's movements during a cracking session and turned them into mathematical equations. (Imaging has not been shown to be very productive in this field, as the crack takes only about 300 milliseconds and is not easily visualized.) The software models demonstrated that pressure shifts in the joint fluid increase pressure on the gas bubbles. Unlike packing material, however, the gas bubbles don't really perforate—they experience a partial collapse but remain suspended in the joint.
So does this solve the mystery surrounding cracked knuckles? Not entirely. Because it was a simulation, there's a possibility of mathematical error. Proponents of alternative theories—that it's not bubbles collapsing but bubbles being created that produce the noise—feel there's more work to be done. We can only hope a complete understanding will come in our lifetime. Fingers crossed. And cracking.